The present invention relates to a process for treating wastewater from lead battery production, and an apparatus for implementing such a process.
Ordinarily, initial charging of the electrodes of a lead battery (which is generally referred to as "formation") is followed by a rinsing process, usually as part of the last wet treatment of the battery production process. At this stage, it is desirable to remove essentially all of the formation acid from the pores of the plates. This washing (with flowing clean water) can be performed on the individual electrodes, before their installation, or in a blocking box, as a group.
The wastewater which is accumulated at this point is contaminated with about 25 g of H.sub.2 SO.sub.4 /l. Such acid wastewater is usually then treated by a two-step neutralization. Milk of lime, caustic soda or sodium carbonate are commonly used as the neutralizing agents.
In the case of precipitation with milk of lime, gypsum is produced which is contaminated with lead and which must therefore be given special handling (as a hazardous waste). High sulfate contents in the wastewater are brought to a concentration corresponding to the solubility of the calcium sulfate. At higher sulfate contents, this lies between 1400 mg/l and 2000 mg/l (approximately). However, because of the risk of concrete corrosion, many governmental authorities require a limit for sulfates in wastewater of between 400 mg/l and 600 mg/l.
Since these values cannot be achieved with a milk of lime process, various, more effective procedures have been proposed. For example, the additional use of calcium aluminate to produce a sulfate precipitation which goes beyond the milk of lime neutralization is suggested in ATV Memorandum M752 (Abwasser bei der Herstellung von elektrischen Akkumulatoren und Primarzellen [Wastewater in the Production of Electric Batteries and Primary Cells], Gesellschaft zur Forderung der Abwassertechnik e.V., St. Augustin, 1992). As the reaction product, this process yields calcium-aluminum sulfate (ettringite) as a voluminous sludge or mud, which increases the quantity of wastes from the wastewater treatment. Also described is treatment of the sulfuric acid discarded from formation, with the aid of distillation. This, however, involves very high energy costs.
Another proposal has been to avoid wastewater and waste products; for example, by collecting the acid which is no longer useful for formation, and in order to lower the sulfate content, to make the collected acid once again available for production as a diluted acid. This is suggested by L. Hartinger, Handbuch der Abwasser und Recyclingtechnik fur die metallverarbeitende Industrie (Wastewater and Recycling Techniques Handbook for the Metal Processing Industry), Carl Hanser Verlag, Munich, Vienna, 1991). However, even in this way, the low limits desired for sulfates in the wastewater are far from reached.
U.S. Pat. No. 4,310,421 suggests dilution as a means for promoting the reuse of wastewater, primarily for the production of positive electrodes for Ni/Cd batteries. As is known, after impregnation with nickel nitrate, sintered electrodes are treated with hot caustic soda to transform the nitrate into an hydroxide. As disclosed, the spent precipitation liquor (with its content of NaOH and NaNO.sub.3, and with a part of its remaining heat) can be reused following treatment in a mixing tank by adding a NaOH solution, with continuous monitoring of its density to a desired concentration, and by then combining the result with fresh precipitation liquor.
EP-OS 454,257 suggests that not only the residual sulfuric acid, but also the sulfuric acid bound to the active masses of used lead batteries, can be transformed into sodium sulfate by reaction with caustic soda and by subjecting the sulfate to an electrolysis in which caustic soda is formed in the cathode space of the electrolysis cell and sulfuric acid is formed in the anode space. After reconcentrating, the sulfuric acid can be used in new batteries. The caustic soda can also be used, again for neutralizing acids and spent lead sulfates.